How long does it take for the content of bone structure to be replaced by minerals, transforming it to rock? Well over six thousand years, that’s for sure. God created the universe in six days, but remember that when the prophets wrote the Bible there was no concept of geological ages. Days were easy for even the uneducated to understand.
Missing the message by dwelling on the minutia.
How would you even know what a day was, if God hadn’t created the Sun until Day 4?
That is dependent on a number of factors including hydrology and the mineral content of percolating water, pH, and the nature of the sediments the bones are buried in.